Lexical selection
Lexical selection is the task of choosing, given several source-language (SL) translations with the same part-of-speech (POS), the most adequate translation among them in the target language (TL). The task is related to the task of word-sense disambiguation. The difference is that its aim is to find the most adequate translation, not the most adequate sense. Thus, it is not necessary to choose between a series of fine-grained senses if all these senses result in the same final translation.
This page has some links to pages about lexical selection in Apertium.
General information:
Current lexical selection module (2012–current)[edit]
The Constraint-based lexical selection module / apertium-lex-tools is made by Francis Tyers and is deployed in the apertium-sh-mk and apertium-kaz-tat language pairs where you can see an example.
This uses a module which runs after bidix, where the bidix output is ambiguous:
morf.analysis | morf.disambiguation | bidix | lexical selection | structural transfer | morf. generation
In a sense, it disambiguates the bidix output (in exactly the same way that morf.disambiguation disambiguates the morf.analysis output).
Some documentation:
- Rule-based lexical selection module
- Learning rules from parallel and non-parallel corpora
- How to get started with lexical selection rules
Old and alternative approaches[edit]
The slr/srl + CG approach (2010-2012)[edit]
This was used in apertium-sme-nob until lately.
This uses a special Constraint Grammar (CG) file which runs after regular morphological disambiguation, but before bidix:
morf.analysis | morf.disambiguation (cg or apertium-tagger) | cg lexical selection | bidix | structural transfer | morf. generation
The CG rules add a number to the lemma of the word if we want a non-default translation, so ^ahte<CC>$ might turn into ^ahte:1<CC>$.
The bidix has entries like
<e> <p><l>ahte<s n="CC"/></l><r>at<s n="cnjcoo"/><s n="clb"/></r></p></e> <e slr="1"><p><l>ahte<s n="CC"/></l><r>og<b/>at<s n="cnjcoo"/><s n="clb"/></r></p></e>
This is pre-processed by an XSLT script, so the file that is given to lt-comp actually contains
<e> <p><l>ahte<s n="CC"/></l><r>at<s n="cnjcoo"/><s n="clb"/></r></p></e> <e R="lr"><p><l>ahte:1<s n="CC"/></l><r>og<b/>at<s n="cnjcoo"/><s n="clb"/></r></p></e>
So if the CG rule fired, and turned ahte into ahte:1, we get "og at" instead of "at".
Downsides with this approach:
- pairs which only want lex.sel require the user to install vislcg3
- developers need to remember when they write the rules that number 1 was "og at" and number 0 was "at", which can get confusing (especially if you decide to change the default) – more points of failure.
- On the other hand side, lexical selection can most often be seen as a / default - special case / dichotomy. A good mode of work is to introduce each rule set with the number array, e.g.: # leat 0 = være, 1 = ha, 2 = måtte («ha å»)
apertium-sme-nob in 2014 switched to bidix before lex.sel (like the lrx-proc method), but still uses vislcg3 rules instead of lrx-proc.
Transfer rule approach (2009)[edit]
You can make transfer rules that does lexical selection. Its not very elegant but it works, to a degree. The drawback is that you:
- get big transfer files
- mix transfer and lexical selection
- must write rules
This is the method used in most trunk pairs.
Lextor (2007)[edit]
Lextor works using statistics and requires 1) slightly pre-processed dictionaries and 2) corpora to train the module. The module is turned off in most cases as it does not provide an improvement over the baseline.
